Multilayer plastic substrates

ABSTRACT

A multilayer plastic substrate. The substrate comprises a plurality of thin film layers of at least one polymer, the plurality of thin film layers being adjacent to one another and having sufficient strength to be self-supporting, wherein the multilayer plastic substrate has an average visible light transmittance of greater than about 80%.

BACKGROUND OF THE INVENTION

The present invention relates generally to plastic substrates which maybe useful in products including, but not limited to, visual displaydevices, and more particularly to multilayer plastic substrates havingimproved light transmittance.

As used herein, the term “(meth)acrylic” is defined as “acrylic ormethacrylic.” Also, (meth)acrylate is defined as “acrylate ormethacrylate.”

As used herein, the term “average visible light transmittance” means theaverage light transmittance over the visible range from 400 to 800 nm.

As used herein, the term “peak visible light transmittance” means thepeak light transmittance over the visible range from 400 to 800 nm.

As used herein, the term “polymer precursor” includes monomers,oligomers, and resins, and combinations thereof. As used herein, theterm “monomer” is defined as a molecule of simple structure and lowmolecular weight that is capable of combining with a number of like orunlike molecules to form a polymer. Examples include, but are notlimited to, simple acrylate molecules, for example,hexanedioldiacrylate, or tetraethyleneglycoldiacrylate, styrene, methylstyrene, and combinations thereof. The molecular weight of monomers isgenerally less than 1000, while for fluorinated monomers, it isgenerally less than 2000. Monomers may be combined to form oligomers andresins but do not combine to form other monomers.

As used herein, the term “oligomer” is defined as a compound molecule ofat least two monomers that maybe cured by radiation, such asultraviolet, electron beam, or x-ray, glow discharge ionization, andspontaneous thermally induced curing. Oligomers include low molecularweight resins. Low molecular weight is defined herein as about 1000 toabout 20,000 exclusive of fluorinated monomers. Oligomers are usuallyliquid or easily liquifiable. Oligomers do not combine to form monomers.

As used herein, the term “resin” is defined as a compound having ahigher molecular weight (generally greater than 20,000) which isgenerally solid with no definite melting point. Examples include, butare not limited to, polystyrene resins, epoxy polyamine resins, phenolicresins, and acrylic resins (for example, polymethylmethacrylate), andcombinations thereof.

There is a need for versatile visual display devices for electronicproducts of many different types. Although many current displays useglass substrates, manufacturers have attempted to produce commercialproducts, primarily liquid crystal display devices, using unbreakableplastic substrates. These attempts have not been completely successfulto date because of the quality, temperature, and permeation limitationsof polymeric materials. Flexible plastic substrates, such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES), have been used in thicknesses from about 0.004inches to 0.007 inches. However, the surface quality of these substratesis often poor, with the surface having large numbers of scratches, digs,pits, and other defects.

In addition, many polymers exhibit poor oxygen and water vaporpermeation resistance, often several orders of magnitude below what isrequired for product performance. For example, the oxygen transmissionrates for materials such polyethylene terephthalate (PET) are as high as1550 cc/m²/day/micron of thickness (or 8.7 cc/m²/day for 7 mil thicknessPET), and the water vapor transmission rates are also in this range.Certain display applications, such as those using organic light emittingdevices (OLEDs), require encapsulation that has a maximum oxygentransmission rate of 10⁻⁴ to 10⁻² cc/m²/day, and a maximum water vaportransmission rate of 10⁻⁵ to 10⁻⁶ g/m²/day.

Barrier coatings have been applied to plastic substrates to decreasetheir gas and liquid permeability. Barrier coatings typically consist ofsingle layer thin film inorganic materials, such as Al, SiO_(x),AlO_(x), and Si₃N₄ vacuum deposited on polymeric substrates. A singlelayer coating on PET reduces oxygen permeability to levels of about 0.1to 1.0 cc/m²/day, and water vapor permeability to about 0.1 to 1.0g/m²/day. However, those levels are still insufficient for many displaydevices.

Additionally, many processes used in the manufacture of displays requirerelatively high temperatures that most polymer substrates cannottolerate. For example, the recrystallization of amorphous Si to poly-Siin thin film transistors requires substrate temperatures of at least160°-250° C., even with pulsed excimer laser anneals. The conductivityof a transparent electrode, which is typically made of indium tin oxide(ITO), is greatly improved if deposition occurs above 220° C. Polyimidecuring generally requires temperatures of 250° C. In addition, many ofthe photolithographic process steps for patterning electrodes areoperated in excess of 120° C. to enhance processing speeds in thefabrication. These processes are used extensively in the manufacture ofdisplay devices, and they have been optimized on glass and siliconsubstrates. The high temperatures needed for such processes can deformand damage a plastic substrate, and subsequently destroy the display. Ifdisplays are to be manufactured on flexible plastic materials, theplastic must be able to withstand the necessary processing conditions,including high temperatures over 100° C., harsh chemicals, andmechanical damage.

Thus, there is a need for an improved plastic substrate for visualdisplay devices, and for a method of making such a substrate.

SUMMARY OF THE INVENTION

The present invention meets this need by providing a multilayer plasticsubstrate. The substrate consists essentially of a plurality of thinfilm layers of at least one polymer, the plurality of thin film layersbeing adjacent to one another and having sufficient strength to beself-supporting, wherein the multilayer plastic substrate has an averagevisible light transmittance of greater than about 80%. The averagevisible light transmittance is typically greater than about 85%, and itcan be greater than about 90%. The peak visible transmittance istypically greater than about 85% and it can be greater than about 90%.

There are typically at least about 50 thin film layers. The number oflayers depends on the thickness of the thin film layers and the desiredoverall thickness of the multilayer plastic substrate. The multilayerplastic substrate is typically at least about 0.001 inches thick, andgenerally at least about 0.004 inches thick. Each thin film layer istypically less than about 50 μm thick.

Polymers include, but are not limited to (meth)acrylate-containingpolymers, styrene containing polymers, methyl styrene containingpolymers, and fluorinated polymers, and combinations thereof. The glasstransition temperature of the at least one polymer is generally greaterthan about 150° C., and it may be greater than about 200° C.

The surface roughness of the multilayer plastic substrate is generallyless than about 10 nm, and it may be less than about 5 nm, or less thanabout 2 nm.

The multilayer plastic substrate can have a refractive index of greaterthan about 1.4 or greater than about 1.5.

The multilayer plastic substrate can include additional layers,including, but not limited to, scratch resistant layers, antireflectivecoatings, antifingerprint coatings, antistatic coatings, conductivecoatings, transparent conductive coatings, and barrier coatings, toprovide functionality to the substrate if desired.

Another aspect of the invention involves a method of making themultilayer plastic substrate. The method includes providing a support,depositing a plurality of thin film layers of at least one polymer onthe support so that the plurality of thin film layers have sufficientstrength to be self-supporting to form the multilayer substrate, andremoving the support from the multilayer substrate, wherein themultilayer plastic substrate has an average visible light transmittanceof greater than about 80%.

The thin film layers can be deposited in a vacuum. One example of avacuum deposition process is flash evaporation. In this method,depositing the plurality of thin film layers includes flash evaporatinga polymer precursor, condensing the polymer precursor as a liquid film,and cross-linking the polymer precursor to form the polymer. The polymerprecursor can be cross-linked by any suitable method, including, but notlimited to, radiation curing, such as ultraviolet, electron beam, orx-ray, glow discharge ionization, and spontaneous thermally inducedcuring.

Alternatively, the plurality of thin film layers can be deposited byextruding or casting a layer of polymer precursor, and cross-linking thepolymer precursor to form the polymer using any suitable cross-linkingmethod.

Accordingly, it is an object of the present invention to provide animproved, multilayer plastic substrate and to provide a method of makingsuch a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of one embodiment of the substrate of thepresent invention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows one embodiment of a multilayer plastic substrate of thepresent invention. The multilayer plastic substrate 100 is formed on asupport 110. After the multilayer plastic substrate is formed, thesupport 110 is removed.

The multilayer plastic substrate of the present invention consistsessentially of a plurality of thin film layers 120 of at least onepolymer adjacent to one another. By adjacent, we mean next to, but notnecessarily directly next to. In most of the multilayer plasticsubstrate, the polymer thin film layers will be directly next to oneanother. However, there can be additional layers intervening betweensome adjacent layers in order to provide additional functionality to themultilayer plastic substrate, as shown in FIG. 1 and described below.

The plurality of thin film layers have sufficient strength to beself-supporting after they are formed. The exact number of thin filmlayers is not critical. It depends on the thickness of each of theindividual thin film layers and the desired overall thickness of themultilayer plastic substrate. There must be enough thin film layers sothat the plurality of thin film layers have sufficient strength to beself-supporting. As used herein, the term self-supporting means thesubstrate can be handled and processed without the need for anunderlying support once the plurality of thin film layers have beendeposited. There are typically at least about 50 thin film layers, moretypically at least about 100 thin film layers. There are generally inthe range of about 500 thin film layers to about 1000 thin film layersor more. Each thin film layer is typically between about 0.05 to about 2μm thick, generally between about 0.2 to about 0.3 μm. If the thin filmlayers are extruded, they are usually thicker, typically up to about 50μm thick, in that case. The multilayer plastic substrate is typically atleast about 0.001 inches thick, and generally at least about 0.004inches thick. A 0.007 inch thick substrate would require about 90 to 350passes of the web past the polymer precursor sources. The multilayerplastic substrate can be flexible or rigid.

The average visible light transmittance of the multilayer plasticsubstrate is greater than about 80%, generally greater than 85%, and itmay be greater than 90%. The peak visible light transmittance isgenerally greater than 85%, and it may be greater than 90%.

The at least one polymer can be any suitable polymer, including, but notlimited to, polymers made from styrene polymer precursors, polymers madefrom methyl styrene polymer precursors, polymers made from(meth)acrylate polymer precursors, for example, polymers made fromhexanedioldiacrylate or tetraethyleneglycoldiacrylate polymerprecursors, and fluorinated polymers, and combinations thereof. Polymersmade from (meth)acrylate polymer precursors work well.

The multilayer plastic substrate can be flexible or rigid. Multilayerplastic substrates made from polymers including, but not limited to,(meth)acrylate polymer precursors will be flexible. One advantage ofmultilayer laminated materials is that they typically have greaterstrength and flexibility than comparable single layer substrates. Amultilayer plastic substrate of the present invention generally hashundreds of cross-linked layers that provide mechanical strength andsufficient rigidity to support the circuitry and devices on the display.

A multilayer plastic substrate made from (meth)acrylate polymerprecursors will have excellent transmission at visible wavelengths.Because polymers made from (meth)acrylate polymer precursors have verylow optical absorption, a multilayer plastic substrate made entirelyfrom such polymers will have high optical transparency, typically anaverage visible light transmittance of greater than about 90%.Multilayer substrates made entirely from fluorinated polymers will alsohave an average visible light transmittance of greater than 90%.Substrates made from styrene and methyl styrene polymers would have anaverage visible light transmittance of about 89%.

The birefringence present in many flexible substrates can be reduced oreliminated with the present invention because the multilayer plasticsubstrate is not mechanically stressed during deposition.

Fully cured layers of polymers made from (meth)acrylate polymerprecursors generally have a refractive index of greater than about 1.5,while fully cured fluorinated polymers generally have a refractive indexof greater than about 1.4. Styrene containing polymers would have arefractive index of about 1.6.

Many optical applications, such as mirrors and reflectors, and displayapplications, such as organic light emitting devices, require substrateswith a surface roughness of less than 2 nm. Surface roughness is theroot mean square of peak-to-valley measurement over a specifieddistance, usually 1 nm. It can be measured using an atomic forcemicroscope or back reflection distribution function. Many substrates donot have the necessary surface smoothness. For example, the surfaceroughness of PET is about 20-50 nm with 100 nm spikes. In contrast,flash evaporated polymer coatings have a very low surface roughness,generally less than about 10 nm, and it may be less than 5 nm, or lessthan about 2 nm. Surface roughness on the order of 1 nm has beendemonstrated. The surface of the multilayer plastic substrate isspecular because of the exceptional smoothness of the polymer layers.

Because the polymer material is highly cross-linked, the multilayerplastic substrate can have a high glass transition temperature andexcellent chemical resistance. The glass transition temperature of theat least one polymer is generally greater than about 150° C., and may begreater than about 200° C.

Polymers including, but not limited to, (meth)acrylates, polycarbonates,polysulfones, polyethersulfones, polymides, polyamides, and polyethernaphthenates have demonstrated excellent resistance to solvents. Thisprovides protection from processing chemicals, ultraviolet lightexposure, and photoresists during lithography processes used tomanufacture flat panel displays and their devices.

The thin film layers that form the multilayer substrate can be depositedby any suitable method, including vacuum flash evaporation, extrusion,or casting. With vacuum flash evaporation, deposition can be performedusing a rotating drum or strap configuration. The polymer precursor isdegassed and metered into a hot tube where it flash evaporates and exitsthrough a nozzle as a polymer precursor gas.

The flash evaporating may be performed by supplying a continuous liquidflow of the polymer precursor into a vacuum environment at a temperaturebelow both the decomposition temperature and the polymerizationtemperature of the polymer precursor, continuously atomizing the polymerprecursor into a continuous flow of droplets, and continuouslyvaporizing the droplets by continuously contacting the droplets on aheated surface having a temperature at or above a boiling point of theliquid polymer precursor, but below a pyrolysis temperature, forming theevaporate. The droplets typically range in size from about 1 micrometerto about 50 micrometers, but they could be smaller or larger.

Alternatively, the flash evaporating may be performed by supplying acontinuous liquid flow of the polymer precursor into a vacuumenvironment at a temperature below both the decomposition temperatureand the polymerization temperature of the polymer precursor, andcontinuously directly vaporizing the liquid flow of the polymerprecursor by continuously contacting the liquid polymer precursor on aheated surface having a temperature at or above the boiling point of theliquid polymer precursor, but below the pyrolysis temperature, formingthe evaporate. This may be done using the vaporizer disclosed in U.S.Pat. Nos. 5,402,314, 5,536,323, and 5,711,816, which are incorporatedherein by reference.

The polymer precursor then condenses on the support as a liquid filmwhich is subsequently cross-linked to form a polymer by any suitablemethod, including, but not limited to, radiation, such as ultraviolet,electron beam, or x-ray, glow discharge ionization, and spontaneousthermally induced curing. This process is capable of depositingthousands of polymer layers at web speeds up to 100 m/min.

Alternatively, after degassing, the polymer precursor can be depositedby extruding, spraying, or casting layers of polymer precursor on thesupport. The polymer precursor is then cross-linked using any suitablemethod, such as those described above.

The functionality of the multilayer plastic substrate can be increasedby the incorporation of functional layers 130, 140, and 150 during thedeposition process. These functional layers 130, 140, and 150 can bedeposited at any time during the deposition process. They can bedeposited below, 130, in between, 140, or on top of, 150, the pluralityof thin film layers 120 of the multilayer plastic substrate, as shown inFIG. 1. As used herein, depositing a coating adjacent to the multilayerplastic substrate includes: depositing the coating on the top layer ofthe multilayer plastic coating; depositing the coating on the multilayerplastic substrate and then depositing additional layers of themultilayer plastic substrate over the coating so that the coating isbetween the layers of the multilayer plastic substrate; and depositingthe coating first and then depositing the layers of the multilayerplastic substrate, and combinations thereof. Functional layers 130, 140,and 150 include, but are not limited to, scratch resistant coatings,antireflective coatings, antifingerprint coatings, antistatic coatings,conductive coatings, transparent conductive coatings, and barriercoatings, and other functional layers. Depositing these additionallayers allows the multilayer plastic substrate to be specificallytailored to different applications. Little or no surface modification isnecessary for deposition of other layers because of the very smoothsurface of the multilayer plastic substrate. Interfaces can be graded tobond all integrated functional layers firmly during the same coating runand pumpdown.

For some applications, it may be important that the presence offunctional layers not reduce the average visible light transmittancebelow 80%, for others, not below 85%, and still others, not below 90%.In others, it may be important that the peak visible light transmittancenot drop below 85%, and for others, not below 90%. In others, it may beimportant that the functional layers not increase the surface roughnessto greater than about 10 nm, for others, not greater than about 5 nm,and for others, not greater than 2 nm.

One type of functional layer that can be included is a barrier coating.One example of a barrier coating is described in application Ser. No.09/427,138, filed Oct. 25, 1999, entitled “Environmental BarrierMaterial for Organic Light Emitting Device and Method of Making,” whichis incorporated herein by reference. The barrier coating can be abarrier stack having one or more barrier layers and one or more polymerlayers. There could be one polymer layer and one barrier layer, therecould be one or more polymer layers on one side of one or more barrierlayers, or there could be one or more polymer layers on both sides ofone or more barrier layers. The important feature is that the barrierstack have at least one polymer layer and at least one barrier layer.The barrier layers and polymer layers in the barrier stack can be madeof the same material or of a different material. The barrier layers aretypically in the range of about 100-400 Å thick, and the polymer layersare typically in the range of about 1000-10,000 Å thick.

The number of barrier stacks is not limited. The number of barrierstacks needed depends on the material used for the polymer of thesubstrate and the level of permeation resistance needed for theparticular application. One or two barrier stacks should providesufficient barrier properties for some applications. The most stringentapplications may require five or more barrier stacks.

The barrier layers should be transparent. Transparent barrier materialsinclude, but are not limited to, metal oxides, metal nitrides, metalcarbides, metal oxynitrides, metal oxyborides, and combinations thereofThe metal oxides include, but are not limited to, silicon oxide,aluminum oxide, titanium oxide, indium oxide, tin oxide, indium tinoxide, tantalum oxide, zirconium oxide, niobium oxide, and combinationsthereof. The metal carbides include, but are not limited to, boroncarbide, tungsten carbide, silicon carbide, and combinations thereof.The metal nitrides include, but are not limited to, aluminum nitride,silicon nitride, boron nitride, and combinations thereof. The metaloxynitrides include, but are not limited to, aluminum oxynitride,silicon oxynitride, boron oxynitride, and combinations thereof. Themetal oxyborides include, but are not limited to, zirconium oxyboride,titanium oxyboride, and combinations thereof.

The polymer layers of the barrier stacks can be made from (meth)acrylatepolymer precursors. The polymer layers in the barrier stacks can be thesame or different.

The barrier stacks can be made by vacuum deposition. The barrier layercan be vacuum deposited onto, or into, the multilayer plastic substrate,or another functional layer. The polymer layer is then deposited on thebarrier layer, preferably by flash evaporating (meth)acrylate polymerprecursors, condensing on the barrier layer, and polymerizing in situ ina vacuum chamber. U.S. Pat. Nos. 5,440,446 and 5,725,909, which areincorporated herein by reference, describe methods of depositing thinfilm, barrier stacks.

Vacuum deposition includes flash evaporation of (meth)acrylate polymerprecursors with in situ polymerization under vacuum, plasma depositionand polymerization of (meth)acrylate polymer precursors, as well asvacuum deposition of the barrier layers by sputtering, chemical vapordeposition, plasma enhanced chemical vapor deposition, evaporation,sublimation, electron cyclotron resonance-plasma enhanced vapordeposition (ECR-PECVD), and combinations thereof.

In order to protect the integrity of the barrier layer, the formation ofdefects and/or microcracks in the deposited layer subsequent todeposition and prior to downstream processing should be avoided. Themultilayer plastic substrate is preferably manufactured so that thebarrier layers are not directly contacted by any equipment, such asrollers in a web coating system, to avoid defects that may be caused byabrasion over a roll or roller. This can be accomplished by designingthe deposition system such that the barrier layers are always covered bypolymer layers prior to contacting or touching any handling equipment.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the compositions and methodsdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. A multilayer plastic substrate consistingessentially of: a plurality of flash evaporated thin film layers of atleast one polymer, the plurality of thin film layers being adjacent toone another and having sufficient strength to be self-supporting,wherein the multilayer plastic substrate has an average visible lighttransmittance of greater than about 80%, wherein the multilayer plasticsubstrate comprises at least about 50 thin film layers, and wherein themultilayer plastic substrate has a surface roughness of less than about10 nm.
 2. The multilayer plastic substrate of claim 1 wherein theaverage visible light transmittance is greater than about 85%.
 3. Themultilayer plastic substrate of claim 1 wherein the average visiblelight transmittance is greater than about 90%.
 4. The multi layerplastic substrate of claim 1 wherein the peak visible lighttransmittance is greater than about 85%.
 5. The multilayer plasticsubstrate of claim 1 wherein the peak visible light transmittance isgreater than about 90%.
 6. The multilayer plastic substrate of claim 1,wherein the multilayer plastic substrate comprises at least about 100thin film layers.
 7. The multilayer plastic substrate of claim 6,wherein the multilayer plastic substrate comprises at least about 500thin film layers.
 8. The multilayer plastic substrate of claim 7,wherein the multilayer plastic substrate comprises at least about 1000thin film layers.
 9. The multilayer plastic substrate of claim 1,wherein the multilayer plastic substrate is at least about 0.001 inchesthick.
 10. The multilayer plastic substrate of claim 1, wherein themultilayer plastic substrate is at least about 0.004 inches thick. 11.The multilayer plastic substrate of claim 1, wherein each thin filmlayer is less than about 50 μm thick.
 12. The multilayer plasticsubstrate of claim 1, wherein each thin film layer is less than about 5μm thick.
 13. The multilayer plastic substrate of claim 1, wherein eachthin film layer is in the range of about 0.05 to about 2 μm thick. 14.The multilayer plastic substrate of claim 1, wherein each thin filmlayer is in the range of about 0.2 to about 0.3 μm thick.
 15. Themultilayer plastic substrate of claim 1, wherein the at least onepolymer is selected from (meth)acrylates, polystyrenes, methylstyrene-containing polymers, fluorinated polymers, polycarbonates,polysulfones, polyethersulfones, polyimides, polyamides, and polyethernaphthenates, and combinations thereof.
 16. The multilayer plasticsubstrate of claim 1, wherein the glass transition temperature of the atleast one polymer is greater than about 150° C.
 17. The multilayerplastic substrate of claim 1, wherein the glass transition temperatureof the at least one polymer is greater than about 200° C.
 18. Themultilayer plastic substrate of claim 1, wherein the multilayer plasticsubstrate has a surface roughness of less than about 2 nm.
 19. Themultilayer plastic substrate of claim 1, wherein the multilayer plasticsubstrate has a refractive index of greater than about 1.5.
 20. Themultilayer plastic substrate of claim 1, wherein the multilayer plasticsubstrate has a refractive index of greater than about 1.4.
 21. Themultilayer plastic substrate of claim 1, wherein the multilayer plasticsubstrate is flexible.
 22. The multilayer plastic substrate of claim 1,wherein the multilayer plastic substrate is rigid.
 23. The multilayerplastic substrate of claim 1, wherein the multilayer plastic substratehas a surface roughness of less than about 5 nm.